1. Field of the Invention
The invention relates to a power conversion system in which a line-commutated power conversion system and a self-commutated power conversion system are coupled.
2. Description of the Related Art
Line-commutated power conversion systems are widely used for conversion from AC power to DC power or from DC power to AC power. By way of example, large-scale systems are used in power conversion systems for DC power transmission.
However, large-scale line-commutated power conversion systems require LC filters, consisting of a reactor and a capacitor, in order to reduce harmonics and compensate for lagging reactive power. In addition to the problem that the space required for the LC filter is large, there are problems of overcurrent in the LC filter caused by the ingress of harmonics from the outside, the occurrence of antiresonance caused by the LC filter and the reactance of the system.
Further, in line-commutated conversion systems, a switching device is commutated using the system voltage. So, there is also a problem in that commutation fails and the system damaged if the voltage of the system should drop or voltage distortion occur during inverter operation.
Active filters and reactive power adjusting devices using voltage type self-commutated power conversion systems, instead of LC filters, have recently appeared, together with advances in self-commutated power conversion technology.
By way of examples, active filters and reactive power adjusting devices, products classed from several MVA to several tens of MVA, have been produced, starting with the device disclosed in the "Toshiba Review" (Vol. 43, No. 4, pp. 339 to 342) for example. The power conversion circuit is of the voltage type, self-commutated type. The primary problem in systems using this voltage type, self-commutated power conversion system is overcurrent protection in the switching device during a DC short-circuit.
The problems associated with a main circuit become particularly apparent at higher capacities. To elaborate, should the GTO (Gate Turn-Off) thyristors be ON at the same time due to a control irregularity or the like, the charge of the DC capacitor is discharged through the GTO thyristors. If there is a rapid rise in the short-circuit current, it is impossible to use the current circuit-breaking function of the GTO thyristor at this time, and there is a risk of overcurrent breakdown of the GTO thyristor. Protective fuses are inserted to prevent the risk.
However, protective fuses are not available for higher voltages and some customers do not like to use them for reasons of guaranteeing long-term reliability. There is a demand for devices to replace them.